Hydrocarbon oils generally contain sulfur compounds. When a hydrocarbon oil is used as a fuel, sulfur in the sulfur compounds is converted to sulfur oxides, which are discharged into the atmosphere. In order to reduce air pollution caused by combustion of a hydrocarbon oil, it is desirable to reduce the sulfur content of a hydrocarbon oil to as low a level as possible. This can be achieved by subjecting the hydrocarbon oil to a catalytic hydrodesulfurization process (HDP).
Catalysts conventionally used for HDP comprise an oxide carrier, such as alumina, magnesia or silica, having supported thereon a metal belonging to Group VI of the Periodic Table (hereinafter simply referred to as a Group VI metal) and a metal belonging to Group VIII of the Periodic Table (hereinafter simply referred to as a Group VIII metal) as active metals. Mo or W is normally used as the Group VI metal, and Co or Ni is normally used as the Group VIII metal.
Addition of phosphorus, boron, etc., to the catalyst has been proposed for improvement of catalytic activity as disclosed in JP-A-52-13503 (the term "JP-A" as used herein means an "unexamined published Japanese patent application). On the other hand, incorporation of a crystalline aluminosilicate into an inorganic oxide carrier is disclosed in JP-A-56-20087. A combination of these techniques is also disclosed in JP-A-61-126196 and JP-A-2-214544.
Conventional methods for incorporating the above-mentioned active metal into a carrier include a so-called impregnation process using an aqueous impregnating solution of a salt of the metal, such as an ammonium salt or a nitrate. Where phosphorus is also to be supported, however, it has been difficult to prepare a catalyst having high contents of a Group VI metal and phosphorus through one-step impregnation on account of the relatively low solubility of a Group VI metal salt into a phosphoric acid solution. Therefore, the impregnation had to be performed in two steps; a carrier is impregnated first with phosphorus and then with a Group VI metal. Otherwise, the amount of the Group VI metal to be supported had to be reduced to an undesirable level.
As restrictions on the sulfur content in gas oil products available on the market have been increased for environmental conservation (e.g., the upper limit of 0.5% by weight was lowered to 0.05% by weight), deeper desulfurization is becoming mandatory, and it has been demanded to establish a technique for removing sulfur-containing compounds which are hardly desulfurized (hereinafter referred to as intractable substances), such as 4-methyldibenzothiophene (4M-DBT) and 4,6-dimethyldibenzothiophene (4,6DM-DBT).
Under these circumstances, JP-A-4-265158 discloses a process for obtaining a catalyst containing more than 20% by weight of a Group VI metal in combination with phosphorus through a single impregnation step. However, the process disclosed needs a considerably high temperature and a long time for preparing an impregnating solution, which has disadvantages for mass production.
The above-mentioned incorporation of crystalline aluminosilicate into an inorganic oxide carrier is aimed at obtaining light oil by cracking of heavy oil and has not been employed for deep desulfurization of a light oil fraction.